1. Field of the Invention
This invention relates generally to hydrodynamic fluid film bearings and, more particularly, to such bearings employing a plurality of thin foils lining the inner surface of a retaining member within which a high-speed rotating shaft is receivable, to establish and maintain a lubricating fluid film, e.g., air, between the foils and the rotating shaft.
2. Related Art
Recent efforts have been made to improve bearings for high speed rotating machinery, such as turbocompressors employed in modern air cycle machines for aircraft cooling and ventilation. Such efforts have led to the development of numerous designs for fluid film hydrodynamic bearings. Generally, fluid film hydrodynamic bearings have been successfully employed in high-speed rotating machines for about the past twenty years. For example, air cycle machines used for aircraft cabin environment control systems utilize fluid film hydrodynamic bearings. Such bearings operate on the principle that a high speed rotating member, such as a shaft, is at least slightly eccentric with respect to rotation about its longitudinal axis. Therefore, if the shaft is enclosed by a close-fitting, compliant, annular element such as a thin foil encased within a stationary retaining member, the eccentricity of rotation within such retaining member will form and maintain a pressurized fluid (e.g., air) film wedge between the shaft and the compliant foil. The high speed rotation of the shaft generates a high pressure in the fluid film wedge, which fluid film supports the load imposed by the shaft. A spring foil, i.e., a resilient backing member, is disposed between the compliant foil and the stationary member (sometimes referred to as a cartridge, retainer or base) to accommodate deflections of the foil resulting from pressurization, centrifugal forces and temperature differentials in order to maintain optimum or at least adequate film layer geometry. The fluid film hydrodynamic bearing desirably has high load capacity and high coulomb damping for suppression of shaft whirl. Providing such characteristics has typically required stringent control of manufacturing tolerances of the fluid film hydrodynamic bearing.
One type of known fluid film hydrodynamic bearing is a multi-pad type as described in U.S. Pat. No. 3,615,121 to Barnett et al and U.S. Pat. Nos. 4,153,315, 4,178,046 and 4,195,395 to Silver et al. Generally, coulomb damping, which is required to suppress whirl of the shaft, is low for such multi-pad bearings, and the low damping has limited the utilization of such bearings. The multi-pad bearings have an iris-type construction and the three aforementioned patents to Silver et al show stiffener elements for the foils. U.S. Pat. No. 4,178,046 discloses a foil bearing in which a plurality of foils is mounted within the retaining member or bushing, each subtending a rotational segment, less than all, of the circumferential or inner surface of the bearing. Each foil is mounted at its midpoint with one side or wing of the foil serving as an underfoil for the overfoil of an adjacent member and the other arm or wing serving as an overfoil for the underfoil of an adjacent member on its opposite side. However, the arrangement of the foils is such that the sliding travel or shifting of adjacent foils (which results from the forces imposed on the foils by the pressurized fluid film generated by the rotating shaft) is in the same direction. Consequently, the relative sliding travel between adjacent foils is the difference between the amount of sliding travel of each foil. This limited relative foil movement contributes to the low coulomb damping characteristic of these multi-pad bearings. Often, in order to compensate for such limited coulomb damping, the art provides multi-pad bearing foils having a preformed diameter (the foil diameter prior to insertion of the shaft into the bearing) which is up to 50% less than the diameter of the shaft which is to be received in the bearing. Consequently, when the shaft is initially mounted within the bearing, the bearing foils maintain a relatively tight grip on the shaft. This results in a high preloading on the shaft and thereby requires a high starting torque. If any type of contaminant, such as water, is present in the bearing, a still higher starting torque is required. Such high starting torque is of course disadvantageous as it stresses the machine which is being used to drive the shaft and may be severe enough to result in inability to start the engine or motor which drives the shaft and/or cause damage to the engine or motor or to drive components.
Improved bearings were provided by the reverse 360-degree multi-layer hydrodynamic fluid film foil bearings described in U.S. Pat. Nos. 4,415,280 and 4,415,281 issued to G. L. Agrawal. In these bearings, two layers of flat compliant foils are arranged to shift in opposite rotational directions and are supported on a layer of corrugated foil which serves as a spring foil and provides high load capacity. Due to the fact that adjacent foils shift in opposite directions, coulomb damping is relatively high because the relative movement between adjacent foils is equal to the sum of the individual foil movements. Accordingly, adequate coulomb damping is attained without the necessity of reducing the preformed radius of the foils to significantly less than that of the shaft. Consequently, preloading imposed on the shaft by the foils is small and the starting torque required is not significantly increased by the bearing. However, because the foils are supported at only one end thereof and extend for 360-degrees around the entire circumference of the inner surface of the stationary member, the foils of these otherwise successful bearings occasionally telescope during assembly and operation. If the foils should telescope during operation, the telescoped foils tighten around the shaft and bind it, and the bearing fails. Further, manufacture of such 360-degree foils is expensive as it requires extensive hand operations to hold the required tolerances.
Hydrodynamic fluid film foil bearings of conical configuration which can serve as thrust bearings to sustain axial loads, that is, loads imposed parallel to the longitudinal axis of the shaft supported by the bearing, are known in the art as evidenced by U.S. Pat. No. 3,382,014 issued to D. J. Marley. The Marley Patent shows in FIG. 2 thereof the overlapping foils of a truncated cone-shaped foil assembly (21) received within a conical recess (19) formed in a bearing housing (17).